If it happened on a
DDO, then variety collectors would probably be less interested in it as if looks less than what a collector would like to see on a variety. A error collector would only accept it as clashed coin. So it would bring no more value with both on it.
Cleaning methods vary. Depending on how deep the clash is. The deeper, the harder they have to dig that clash mark out. after a few cleanings they devices edges will loose detail. The field is the outside of the die, the devices deeper into the die. Cleaning clash marks usually affects the fields. Sometimes deeper. What they use, I can only speculate. But the way this coin looks like they used a steel brush. But other methods have been use though.